The need for low-emissions fuels has created an increased demand for light olefins used in alkylation, oligomerization, MTBE and ETBE synthesis processes. In addition, a low cost supply of light olefins, particularly propylene, continues to be in demand to serve as feedstock for polyolefin production, particularly polypropylene production.
Fixed bed processes for light paraffin dehydrogenation have recently attracted renewed interest for increasing olefin production. However, these types of processes typically require relatively large capital investments and high operating costs. It is therefore advantageous to increase olefin yield using processes that require relatively small capital investment. It would be particularly advantageous to increase olefin yield in catalytic cracking processes.
While conventional fluid catalytic cracking processes are suitable for producing conventional transportation fuels, these fuels are generally unable to meet the more demanding requirements of low emissions fuels and chemical feedstock production. To augment the volume of low emission fuels, it is desirable to increase the amounts of light olefins, such as propylene, iso- and normal butylenes, and isoamylene. The propylene, isobutylene, and isoamylene can be reacted with methanol to form methyl-propyl-ethers, methyl tertiary butyl ether (MTBE), and tertiary amyl methyl ether (TAME). These are high octane blending components which can be added to gasoline to satisfy oxygen requirements mandated by legislation. In addition to enhancing the volume and octane number of gasoline, they also reduce emissions. It is particularly desirable to increase the yield of ethylene and propylene, which are valuable chemical raw materials. Conventional fluid catalytic cracking does not produce large enough quantities of these light olefins, particularly ethylene. Consequently, there exits a need in the art for methods of producing larger quantities of ethylene and propylene for chemicals raw materials, and other light olefins for low-emissions transportation fuels, such as gasoline and distillates.
A problem inherent in producing olefin products using FCC units is that the process depends upon a specific catalyst balance to maximize production. In addition, even if a specific catalyst balance can be maintained to maximize overall olefin production, olefin selectivity is generally low due to undesirable side reactions, such as cracking, isomerization, aromatization and hydrogen transfer reactions. Therefore, it is desirable to maximize olefin production in a process that allows a high degree of control over the selectivity of C.sub.2, C.sub.3, and C.sub.4 olefins.